A PET (Positron Emission Tomography) apparatus will be described as an example of nuclear medicine diagnostic apparatus, i.e. ECT (Emission Computed Tomography) apparatus. The PET apparatus is constructed to detect a plurality of γ-rays generated by annihilation of positrons, and to reconstruct a sectional image of a patient only when a plurality of detectors simultaneously detect the γ-rays.
Specifically, a patient is medicated with a radioactive drug including a positron-emitting radionuclide, and detectors consisting of numerous detecting element (e.g. scintillator) groups detect pair annihilation γ-rays of 511 KeV released from the patient medicated. And when two detectors detect γ-rays within a definite period of time, they are counted as one pair of annihilation γ-rays detected as a coincidence, and a pair annihilation generating point is determined to exist on a straight line linking the detector pair having detected them. Such coincidence information is accumulated and reconstruction is carried out to obtain a positron-emitting radionuclide distribution image (i.e. a sectional image).
At this time, image resolution of the sectional image is improved by increasing the number of scintillators to obtain more particular γ-ray detecting positions on the detectors, combining them with photomultiplier tubes (PMT) capable of detecting positions, and discriminating γ-ray detecting positions as individual scintillator elements to increase γ-ray detecting accuracy. So, the number of scintillators is increased to increase discriminating capability. In recent years, in particular, DOI detectors have been developed, which have scintillators laminated also in a depth direction to be capable of discriminating light source positions having caused interaction in the depth direction (DOI: Depth of Interaction).
To discriminate γ-ray incident positions, a two-dimensional position map prepared beforehand is used. The two-dimensional position map is drawn by centroid calculation of electric signals acquired with light sensors represented by position detecting type photomultiplier tubes, to calculate two-dimensional coordinates (X, Y) relating to events of detecting γ-rays. Further, this two-dimensional position map is obtained by emitting γ-rays in uniform parallel beams to the detectors, repeating the above operation while the γ-rays are detected, and integrating two-dimensional coordinates on a two-dimensional plane. These are drawn as a distribution with peaks corresponding to respective positions of scintillator elements (crystal elements). FIG. 9 shows a two-dimensional position map in the case of a DOI detector having four layers of scintillators laminated in the depth direction. The positions indicated by white circles (shown as “◯” in FIG. 9) are scintillators in the first layer (written “1st Layer” in FIG. 9). The positions indicated by white rhombuses are scintillators in the second layer (written “2nd Layer” in FIG. 9). The positions indicated by white double octagons are scintillators in the third layer (written “3rd Layer” in FIG. 9). The positions indicated by white rectangles (shown as “□” in FIG. 9) are scintillators in the fourth layer (written “4th Layer” in FIG. 9). Incident positions of actually incident γ-rays can be discriminated by referring to a look-up table (LUT) having each position in the two-dimensional position map corresponding to each scintillator, and referring to the two-dimensional position map.
Incidentally, where a plurality of scintillators are arranged in three dimensions as in the DOI detector, diffusion is provided by combination of a light reflective material and a light transmissive material, for example, between adjoining scintillators, so that positions do not overlap in the two-dimensional position map. Further, a technique of correcting the two-dimensional position map has been introduced, which carries out a statistical clustering process in order to increase the discriminating capability still further (see Patent Document 1, for example).
On an actual two-dimensional position map, peaks of signal strength appear in a grid form. If to which of the positions (rows and columns) on the two-dimensional position map a peak belongs is known, it is possible to discriminate which scintillator element (crystal) in a scintillator block (crystal block) incidence has occurred, and whether light has been emitted from that crystal. Therefore, it is necessary to set boundaries for area division of the entire two-dimensional position map into spheres of influence of respective peaks. As a result, each point on the screens of light sensors is in the sphere of influence of one of the peaks.